1. Field of the Invention
This invention relates to an absolute encoder which is provided with a code plate having a 1-track type absolute pattern and which reads absolute positional information of a detector section around the code plate from the pattern, and more particularly to a method of improving resolution of the absolute encoder by subdividing a minimum read unit of the absolute pattern (interpolation processing).
2. Related Background Art
An absolute encoder is a measuring device in which sensors located on a detector section of the encoder read special detected patterns formed on a code plate and thus generate a signal which indicates an absolute position of a detector section around a code plate. The detected patterns indicate each of different address information of various positional relationships between the code plate and the detector section. In prior art of absolute encoders, a multi-track type encoder is well known in which a plurality of digital repeating patterns (incremental patterns) having different pitches respectively are arranged on the code plate of the encoder in parallel and addresses of the absolute position are indicated by each signal received from the same phase position of the plural patterns.
On the other hand, as seen in Japanese Patent Application Laid-Open No. 1-152314, recently, a 1-track type encoder has been well researched in which addresses of an absolute position are indicated by detecting a plurality of phase positions on a detected pattern (a 1-track type absolute pattern) with a minimum read unit according to a special sequence. Compared with the multi-track type encoder, the 1-track type absolute encoder is easy for construction and adjustment of a sensor and suitable for miniaturization of a code plate.
It is reported that a 1-track type absolute encoder is improved through this research and development in which incremental patterns are arranged in parallel with an absolute pattern on the code plate. Prior art, shown in Japanese Patent Application Laid-Open No. 2-35314, for example, is known in which read time of a 1-track type absolute encoder is controlled by detected signals or resolution of the encoder is improved by combining an address obtained from a 1-track type absolute pattern with signals (0, 1) received from incremental patterns.
FIG. 6 is a typical plan view of the absolute encoder whose resolution is improved.
As shown in FIG. 6, the first incremental track H1 with pitch .lambda., the second incremental track H2 with pitch .lambda./2 and the third incremental track H3 with pitch .lambda./4 are formed on the code plate A in parallel with the 1-track type absolute pattern P in which the length of a minimum read unit is .lambda.. Photo sensors S1-S4 and U1-U3 are mounted on a detector section B respectively to detect four minimum read units next to one another on the track P and tracks H1-H3.
The track P represents all 4-bit codes in a cycle sequence which moves from a symbol .gradient. in a clockwise direction; and white and black sections mean 0 and 1 respectively, wherein the sequence is EQU 0000100110101111.
Each of the 4-bit codes
in the sequence differs from the others. As shown in FIG. 6, if the detector section B is moved in a clockwise direction in succession, binary digit codes which are obtained from the outputs of the photo sensors S1-S4 are 16 types: 0000, 0001, 0010, 0100, 1001, 0011, 0110, 1101, 1010, 0101, 1011, 0111, 1111, 1110, 1100, and 1000 every .lambda. movement quantity. The codes discriminate each of 16 absolute positions.
On the other hand, from the outputs of the photo sensors U1-U3 which read the tracks H1-H2, in which the white and the black sections are 0 and 1 respectively, 3-bit codes: 111, 110, 101, 100, 011, 010, 001, and 000 every .lambda./8 in a clockwise direction in all .lambda. sections on the code plate are obtained. The multi-track type absolute encoder is composed of tracks H1-H3 and photo sensors U1-U2.
A total of each 7-digit different absolute positional information items, in which the detector section B can take all positional relationships every .lambda./8 on the code plate A, can be obtained in such a manner that codes which are obtained from the photo sensors U1-U3 as high order 3 digits are combined with codes obtained from the photo sensors S1-S4 as low order 4 digits.
If all cycle sequences which can discriminate 128 absolute positions per cycle every .lambda./8 are employed and a 1-track type absolute pattern which expresses the sequences with the white and the black sections is applied to the track P, an absolute encoder of the same resolution can be obtained without tracks H1-H3. However, if a minimum read unit of the 1-track type absolute pattern is minimized the sensor is also required to be minimized and reading precision (reliability) is hard to preserve owing to constraint of processing and mounting of the sensor. In an absolute encoder having additional three tracks H1-H3, as shown in FIG. 6, since a minimum section is discriminated by incremental patterns, if a detection method which utilizes a characteristic of the cycle (rule) is employed, far higher reading precision can be obtained than that of the above-described pure 1-track type encoder.
FIGS. 7A to 7C illustrate examples of detection methods utilizing a characteristic of this pattern. FIG. 7A is a vertical sectional view of a typical detector section. FIG. 7B is a line view of the amount of receiving light from the detector section. FIG. 7C is a line view of incremental signals obtained from the amount of receiving light. This art has been widely used in a multi-track type absolute encoder in general.
Referring to FIG. 7A, incremental tracks H with pitch .lambda. are formed on the code plate A. The detector section is composed of a light F for pattern reading, a collimating lens L1 for obtaining parallel light, an index scale (mask) M whose pattern of pitch like track H is formed for 5 pitches in length, a condensing lens L2 for condensing, and a sensor U.
In the above-described detector section B, light radiated from light F changes into parallel light through the lens L1. The light which penetrates a 5-pitch overlapping section between the scale M and the track H is condensed into the sensor U through the lens L2.
In a 1-pitch movement of detector section B around the code plate A, as shown in FIG. 7B, a level of light detected from the sensor U goes up and down in a triangular wave-shaped manner and the wave forms peaks and bottoms have a high signal-to-noise ratio. Also, by a circuit which compares the amount of light detected with a reference level at a broken line shown in FIG. 7B, incremental signals which discriminate two conditions on 1-pitch can be obtained (FIG. 7C).
In this way, small pitches can be read without miniaturization of the sensor. In the absolute encoder as shown in FIG. 6, if this method of detection is applied to tracks H1-H3, the resolution of the encoder can be increased to 2.sup.n, wherein n represents 3 incremental tracks.
If a method of an absolute encoder as shown in FIG. 6 is considered further, an absolute encoder having far higher resolution can be obtained by adding a track H4 with pitch .lambda./8 and a track H5 with pitch .lambda./16 to the code plate A. However, if the number of tracks is increased, since the code plate A is gradually maximized and the number of sensors is also increased, the tracks and sensors are hard to mount and adjust on the encoder and most of the merits of 1-track type absolute patterns disappear. It is pointed out that the absolute encoder is preferably designed by employing the multi-track type from the beginning.